Peptic ulcer bleeding often stops spontaneously but rebleeding may be catastrophic. Emergency surgery carries risks so safe medical therapies are needed. Since platelet function and plasma coagulation are both pH sensitive and since pepsin lyses clot at low pH the maintenance of gastric pH close to neutrality might influence rebleeding rates. Previous trials with H2 antagonists have been inadequate although a 1985 meta-analysis did support an important clinical effect. We report here a large multicentre trial of famotidine in ulcer bleeding. 1005 patients admitted to one of sixty-seven hospitals in the UK or Eire with haemorrhage from peptic ulcer with endoscopic signs of oozing, black slough, fresh clot or visible vessel were randomly allocated to famotidine (10 mg bolus followed by 3.2 mg/h intravenously) or matching placebo for 72 h. This famotidine regimen had previously been shown to maintain pH near 7 in such patients. 497 patients received famotidine and 508 placebo. The treatment groups were similar in respect of age, sex, ulcer site, and signs and severity of bleeding. Case fatality (6.2% famotidine vs 5.0% placebo), rebleeding (23.9% vs 25.5% placebo), and surgery (15.5% vs 17.1% placebo) rates were not significantly different between the two groups. This trial suggests that potent inhibition of gastric secretion does not influence the natural history of peptic ulcer haemorrhage.